An air preheater, often referred to as a rotary air preheater, transfers heat from a hot gas stream such as, for example, flue gas leaving a boiler, to one or more colder gas streams such as, for example, a combustion air stream entering the boiler. Heat is transferred from the hot gas stream to the colder gas stream(s) through a regenerative heat transfer surface in a rotor of the air preheater, which turns continuously through both the hot and colder gas streams. Hereinafter, the hot gas stream shall be referred to as the flue gas stream while the colder gas stream(s) shall be referred to as the combustion air stream(s) or air stream(s).
The rotor, which is packed with the regenerative heat transfer surface, is divided into compartments by a number of radially extending plates referred to as partition walls or diaphragms. The compartments hold baskets in which the regenerative heat transfer surface is contained.
The air preheater rotor is further divided into a flue gas passage and one or more air passages by sector plates. From a temperature standpoint, the air preheater may also be considered as being divided into descriptive regions commonly referred to as hot and cold ends. For a conventional rotary air preheater, the hot end region describes all stationary and rotating components in general proximity to the axial end where the hot flue gas enters the air preheater. The cold end refers to the general region at the axial end opposite the hot end, where the cold combustion air enters the air preheater. In a typical installed rotary air preheater, rigid or flexible radial seals are mounted at the hot and cold end edges of the rotor diaphragms and in close proximity to their respective hot and cold end sector plates. The radial seals help to minimize the leakage of air to the flue gas stream, as well leakage between multiple air streams. Similarly, rigid or flexible axial seals mounted on outboard edges of the diaphragms are in close proximity to axial seal plates mounted on an inner surface of the housing and minimize leakage therebetween. The axial seals and axial seal plates are located in the general region between the hot and cold ends of the air preheater.
In typical installed air preheaters, the number of diaphragms and the width of the sector plates and the seal plates are such that only one radial seal and one axial seal are disposed proximate to the respective plate at any one time. These seals are proximity seals and are not designed to contact the sealing surface of the sector plates or axial seal plates. They are, in fact, typically installed with predetermined clearance gaps to their respective sealing plates. In the case of the cold end radial seals and the axial seals, the clearance gaps are used to avoid relatively substantial seal contact and wear that would result from the operating thermal deformations of the rotor diaphragms. At both the cold end radial seals and the axial seals, operating thermal deformations tend to move the seals closer to their respective sealing plates. Thus, predetermined seal clearance gaps at the time of installation are typically reduced during operation, and the leakage at these seals is passively minimized. In the case of the hot end radial seals, thermal deformations tend to move the outboard ends of these seals away from the hot end sector plates. Consequently, thermal deformations can cause an increase in the leakage past the hot end radial seals, where the amount of leakage is dependent on the pressure differential between the air and gas streams as well as the thermally enlarged gaps between the seals and the sector plates.
To minimize hot end radial seal leakage, it is often advantageous to make use of automatically actuated hot end sector plates that enable the aforementioned outboard leakage gaps to be reduced during operation. Such adjustments are achieved utilizing a mechanical drive system located near the outboard end of the hot end sector plates. In order to achieve proper on-line adjustment, sector plates are often fitted with rotor position sensing devices. Typically, sensing devices contain mechanical limit switches and a sensor rod and, working in conjunction with the sector plate drive system, rely on momentary contact with a sensing surface on the rotor to determine rotor position. Given a fixed dimensional relationship between the rotor sensing surface and the edges of the hot end radial seals, the detection of this sensing surface enables the sector plate drive system to position the sector plate closely to the edges of the hot end radial seals. In this way, hot end radial seal leakage can be minimized.
Over the long term, repeated contact with the rotor sensing surface eventually leads to failure of the limit switches or wear of the sensor rod. Failure of the limit switches and wear of the sensor rod may result in the need for frequent maintenance.